Voltage margining is a method for testing the ability of a transmitter or receiver to tolerate poor or marginal signal quality by purposely reducing the amplitude of voltage swings of an input signal. One approach to voltage margining is to use a high-speed tester. A tester should be able to generate a pattern of logical ones and zeros at high speed, such as for example at a speed of 6.4 GTS (Giga-Transfers-per-Second). The tester should also be able to adjust the amplitude of the input voltage swings. However, such a tester may be expensive.